1. Field of the Invention
The present invention relates to a positioning apparatus which positions a stage, an exposure apparatus having the positioning apparatus, and a device manufacturing method using the exposure apparatus.
2. Description of the Related Art
An exposure apparatus which transfers the pattern of an original onto a photosensitive agent applied on a substrate is employed to manufacture a device such as a semiconductor device, liquid crystal display device, or thin-film magnetic head by photolithography.
Nowadays, to obtain a higher resolving power, the numerical aperture of a projection optical system is increasing. Along with an increase in the numerical aperture of the projection optical system, the depth of focus of the projection optical system is decreasing. It is therefore becoming important to improve the focus accuracy.
Under the circumstance, the exposure apparatus is provided with an auto focus mechanism which measures the surface position of a substrate (the position of the projection optical system in the optical axis direction) by an oblique-incidence position measuring device, and focuses the substrate surface onto the image plane position of the projection optical system.
Also, an imaging error due to deformation of the original is becoming non-negligible. To cope with this situation, an oblique-incidence position detection system measures the shape of the original pattern surface while the original is held by the original stage. Based on the measurement result, the amount of change in imaging plane position is calculated. The imaging characteristic of the projection optical system, or the focus position of the substrate or original is controlled so that this change amount is corrected.
The exposure apparatus generally uses a laser interferometer and reflecting mirror to measure the position of a stage. Japanese Patent Laid-Open No. 2002-319541 discloses an arrangement in which a laser interferometer is used to measure the position of a stage. In the position measurement using a laser interferometer, the laser wavelength changes due to slight fluctuations in atmospheric pressure, temperature, and humidity, leading to a measurement error. To solve this problem, the arrangement disclosed in Japanese Patent Laid-Open No. 2002-319541 controls the environment in the exposure apparatus by covering it with a chamber, and corrects fluctuations that remain uncorrected by this control, using the measurement values obtained by a barometer, thermometer, and hygrometer.
Japanese Patent Laid-Open No. 2006-120699 discloses an apparatus which measures the position of a stage using a plurality of laser interferometers. In this arrangement, the used laser interferometers are switched.
To perform position measurement by switching between the used laser interferometers, it is necessary to prevent any shift in measurement value between before and after switching. For this purpose, an overlapping interval in which the plurality of laser interferometers can simultaneously measure the position of a stage must be provided to allow a laser interferometer after switching to take over the measurement value obtained by that before switching.
Correction to reduce a measurement error due to fluctuation in wavelength as described above can be performed assuming the optical path length in resetting a laser interferometer as a base point. An apparatus which does not switch between laser interferometers continuously performs correction unless it stops due to, for example, an error. However, when the laser interferometers are switched, the position information must be set in a laser interferometer after switching for every switching. Because it is difficult to switch between the laser interferometers at exactly the same position, the optical path length serving as the base point of wavelength correction changes upon this switching. Unless this change amount is taken into consideration, it causes a measurement error, resulting in a positional shift.
When a moving stage is controlled in the pitching direction using a laser interferometer (to be referred to as a pitching interferometer hereinafter), the measurement result obtained by the pitching interferometer can have an error due to the influences of the temperature, humidity, and atmospheric pressure. This error changes the orientation of the stage in the pitching direction. In switching from one laser interferometer to another one, position information including an error is set in this other laser interferometer. This can cause an error in the position measurement value of the stage. This error can occur not only due to a change in orientation in the pitching direction but also due to a change in orientation in the yawing direction.
The above-described problem can be posed even when a measuring device which measures a relative position by another scheme in place of laser interferometers is used.